Reviving a 30-year-old controversy, Berkeley scientists have pinned down the precise time when a monster comet or asteroid slammed into Earth and, they say, wiped out the dinosaurs in a global mass extinction.

Paul Renne, a physicist at the Berkeley Geochronology Center, and his colleagues reported Thursday in the journal Science that with new and highly advanced dating technology they have calculated the impact to 66,038,000 years ago, give or take only 11,000 years.

In geologic terms, that's "to within a gnat's eyebrow," Renne said.

And by that standard, both the impact and the mass extinction occurred at virtually the same time - within less than 33,000 years of each other, Renne and his international team of scientists maintain.

Scientists have debated the cause of the dinosaurs' demise for more than 30 years.

Some argue the impact caused a deadly global climate change when immense clouds of dust and debris blotted out all sunlight for thousands - perhaps millions - of years. Others insist it wasn't the crash that caused the mass extinction, but a series of monstrous volcanic eruptions in India that blackened the skies and left the evidence in an immense basaltic region of India called the Deccan Traps.

Start of a controversy

The often bitter controversy goes back to 1980, when Walter Alvarez, a UC Berkeley geologist, and his father Luis, a UC Berkeley Nobel laureate, first proposed that a huge object falling from outer space hit Earth some 65 million years ago and caused the dinosaurs' extinction.

The black clouds of dust and rocky debris containing the rare space element iridium changed Earth's climate for months, and drove three-quarters of the world's plant and animal life to extinction, including all the dinosaurs and almost all the species of marine plankton, the Alvarez group maintained.

Supporting their claim, other scientists later discovered the site of the impact: a huge crater 110 miles wide, most of it deep in a seabed off the coast of the Yucatan near the Mexican village of Chicxulub. The Alvarez theory has been widely accepted ever since.

Now, Renne and his colleagues in Scotland and the Netherlands have developed a high-precision dating technology. And they used it to date samples of shocked quartz and tiny glass tektites blown out from the impact onto Haiti and to the Hell Creek formation of Montana where dinosaur fossils abound.

The age of the Yucatan crater and the age of the mass extinction coincided within 33,000 years, they reported in Science.

The technology involved a new way to calculate the precise rate that a radioactive form of the element argon decays to its stable form, a specialty of Renne's research center known as Argon-Argon dating.

Support for Renne

Strong support for the Renne team's conclusions came in the same issue of Science from a major expert on dating technology, Heike Pälike of the University of Bremen, Germany.

He called Renne's methods state-of-the-art and said they refute a rival theory that insists the Chicxulub impact occurred at least 300,000 years before the mass extinction and the death of the dinosaurs.

At UC Berkeley, however, paleontologist Kevin Padian, a noted expert on dinosaur evolution, challenged the Renne team's link between the impact and the mass extinction.

He agreed that Renne's dating system is a "fine advancement," but he insisted that "as far as the actual fossil record goes, the speculations in this paper make no sense at all."

Padian said that dinosaur diversity in the Montana region had already been declining for millions of years before the Chicxulub impact, and that only three types of those creatures - including the Tyrannosaurus - remained on Earth by the time of the mass extinction.

One of the main opponents of the Alvarez theory and of Renne's dating results is Gerta Keller, a noted Princeton geophysicist. She has argued for nearly 30 years that the ancient volcanoes of the Deccan Traps caused the mass extinction and in an e-mail Thursday she insisted that Renne's own dates continue to show that the Chicxulub impact occurred at least 300,000 years before the mass extinction.

Giving ground

All sides agree that the dinosaurs died during the ancient geologic period now called the Cretaceous-Paleogene boundary some time around 65 million years ago. And both camps are now willing to give some ground. Renne concedes that the Deccan volcanoes may have played some minor role in the mass extinction, but still insists that the impact was the "tipping point."

Keller holds to her Deccan Trap hypothesis but has conceded the impact that blasted out the Chicxulub crater might also have been involved at some point in the mass extinction.